The proper differentiation of T helper (Th) cells to Th1 or Th2 cells in response to antigen stimulation is critical to initiate a cellular or a humoral immune response, respectively. Multiple factors, including cytokines, the nature and the dose of antigen, costimulators, and the strength of interaction between antigen presenting cells (APC) and T cells, participate in the regulation of Th cell differentiation. The signals delivered by these factors eventually direct the expression of IFN-gamma or IL-4, the hallmark cytokines of Th1 and Th2 effector cells, respectively. Recently, we have revealed a novel role of MHC class II transactivator (CIITA) as a repressor of IL-4 gene expression. We showed that Th1 cells lacking CIITA produced IL-4 in addition to IFN-y. Moreover, introduction of CIITA to a Th2 cell line resulted in down-regulation of IL-4 gene expression. Although our data suggest that CIITA regulates IL-4 expression, the underlying mechanism governing this regulation remains to be elucidated. Our long-term goal is to understand the role of CIITA for the programming of Th cell development. To achieve this, however, it is obligatory to study the regulation of CIITA gene expression during Th cell development. Our hypothesis is that the proper expression of CIITA during Th1 cell development is required to repress IL-4 gene transcription. This paradigm then predicts that constitutive expression of CIITA in vivo would result in the repression of IL-4 from Th2 cells. Accordingly, our alms are as follows: Aim 1 is to determine whether constitutive expression of CIITA represses IL-4 production from developing Th2 cells in vivo by using transgenic mice expressing CIITA in CD4 T cells. In Aim 2, we will study CIITA gene regulation during Th cell differentiation from the naive to the effector T cell stage. To do this, we will generate the knock-in mice expressing a GFP reporter in the CIITA locus. This will allow us to monitor the specificity and the dynamics of CIITA expression during Th cell differentiation in vivo. In Aim 3, we will clone the isoform and the promoter of CIITA that is expressed in T cells. Aim 4 focuses on the analysis of the CIITA promoter to determine the minimal promoter that is sufficient for I cell expression. The results from these experiments will help us to unravel the complexity and the role of CIITA in the regulation of the IL-4 gene during T cell differentiation.